Ecosystems, Matter & Energy — Chapter review · Life Science & Scientific Inquiry, Grade 7

Five focused units, a three-session investigation, chapter review, and chapter quiz.

Review the key ideas

Energy Flow, Food Chains & Food Webs

An ecosystem includes organisms and their physical environment. Producers build organic molecules using an energy source, commonly sunlight in the ecosystems studied here. Consumers obtain organic matter by eating organisms or their products. Decomposers use dead material and wastes, transferring matter into other forms. These are functional roles, not ranks of importance.

Energy Pyramids & Transfer Efficiency

An energy pyramid represents measured or modeled energy at successive feeding positions for a defined area and interval. It commonly decreases upward because organisms use energy in their own processes and some matter is not consumed or assimilated. Much transferred energy eventually spreads as heat rather than becoming food for the next position.

Carbon, Water & Cycling Matter

Matter moves among organisms, air, water, and soil. Carbon dioxide can supply carbon for photosynthetic food production; feeding transfers organic matter; respiration and decomposition return carbon to other forms. Some reservoirs store carbon for long periods, so a cycle does not mean every atom completes the same path quickly.

Decomposition, Soil & Nutrient Availability

Soil is a changing mixture of mineral particles, organic material, water, air spaces, and organisms. Dead leaves are not simply converted into new plants. Decomposers process organic material, release energy through their own metabolism, and move nutrients into forms that can enter other pathways. Plants need suitable resources as well as an energy source.

Habitats, Resources & Population Change

A population includes individuals of the same species in a defined area and interval. A community includes populations of different species. A habitat is the setting supplying conditions and resources for an organism. Biotic factors involve organisms; abiotic factors include physical conditions such as water, light, and temperature.

Problem 1

Illustrative web: grass feeds insects and rabbits; insects and seeds feed mice; mice and rabbits feed foxes. An insect decline is recorded, but other resource counts are missing. Trace one affected pathway and explain why the exact mouse or fox change cannot be determined.
Answer and explanation

The insect decline can affect the mouse pathway and indirectly foxes, but mice also have seeds and foxes also have rabbits. The web does not give dependence strengths, alternative resource changes, movement, or timing. It supports a conditional mechanism, not an exact population forecast.

Follow insect→mouse→fox under the stated arrows.

Identify seeds as another mouse resource and rabbits as another fox resource.

Request counts, rates, and interval information.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 2

Why is a decomposer not outside the food system?
Answer and explanation

It obtains and transfers organic matter and energy from dead material and wastes within connected pathways.

Problem 3

Illustrative energy model: producers provide 2,000 units; first consumers receive 300; second consumers receive 60 during the same interval. Calculate both transfer percentages and explain why the ten-percent shortcut and an energy-destruction claim are wrong.
Answer and explanation

The two modeled transfers are 15% and 20%. They are not both ten percent. Lower energy reaching the next position does not mean energy was destroyed; organisms use energy and transfer it into other pathways and heat. The model does not give the full separate budget or exact organism counts.

300/2,000×100=15%.

60/300×100=20%.

Identify other energy pathways and the stated boundary.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 4

Why can a few trees support many insects without contradicting energy flow?
Answer and explanation

Individual count differs from energy amount; organisms vary greatly in size and available energy.

Problem 5

Illustrative pond interval: rain adds 40 L; runoff adds 20 L; evaporation removes 25 L; outflow removes 15 L. Other pathways are excluded. Calculate storage change, label inputs and outputs, and explain the conservation distinction.
Answer and explanation

The modeled pond gains 20 L. Rain and runoff are inputs, while evaporation and outflow cross outward. The changing pond amount is consistent with matter conservation because the pond is only one boundary. The result assumes the stated pathways are complete for the interval and does not predict future weather or every real pond exchange.

Inputs: 40+20=60 L.

Outputs: 25+15=40 L.

Change: 60−40=+20 L.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 6

Does sunlight supply the carbon atoms in sugar?
Answer and explanation

No. Carbon dioxide supplies carbon matter in the plant model; light supplies energy.

Problem 7

Illustrative dry-material study over the same interval: matched samples each start at 20 g; A ends at 14 g and B at 18 g. Moisture treatment differs; other conditions are stated matched. Calculate mass losses and explain the supported pattern with a boundary limitation.
Answer and explanation

A shows 6 g loss and B 2 g, so A lost 4 g more in this illustrative comparison. This is consistent with a treatment-related decomposition question under the stated matching. It does not identify every organism or destination of matter. Check replication, dry-mass method, and material transfers before generalizing to every soil.

A loss: 20−14=6 g.

B loss: 20−18=2 g.

Compare dry-mass method and missing destinations.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 8

Why are spaces in soil relevant?
Answer and explanation

They can hold air and water and affect movement and root conditions, rather than being biologically meaningless empty areas.

Problem 9

Illustrative interval: population begins at 50; births add 12; immigration adds 5; deaths remove 8; emigration removes 4. No other pathways are stipulated. Calculate ending count and explain why a before-after count alone cannot identify all pathways.
Answer and explanation

The modeled population ends at 55, a net increase of five. The same net change could result from other combinations of births, deaths, and movement. Before-after counts alone do not identify the mechanism; pathway observations, boundaries, and detection methods are needed. This model does not establish a permanent habitat capacity.

Gains: 12+5=17.

Losses: 8+4=12.

End: 50+17−12=55.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 10

Why should a count include area and effort?
Answer and explanation

Without them, equal or different counts might reflect different survey scope rather than biological change.

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